Laser welding is rapidly becoming a common way to manufacture small electronic devices, including, in particular, optoelectronic devices.
More particularly, laser welding is commonly used to fasten together two metal elements, by subjecting a contact point to laser radiation such that a spot weld is formed.
Several different types of spot welds can be formed, depending on the nature of the elements which are being connected. By way of example but not limitation, a so-called "butt weld" is shown in FIG. 1; a so-called "lap weld" is shown in FIG. 2; a so-called "fillet weld" is shown in FIG. 3; a so-called "hybrid fillet-lap weld" is shown in FIG. 4; and a so-called "butt-lap weld" is shown in FIG. 5.
With conventional laser welding, the laser radiation is typically provided in the form of a 1 millisecond pulse from a 0.2-20.0 joule laser, operating at a wavelength of 0.3-10.0 microns. Currently, the most popular laser for laser welding is believed to be a Neodymium YAG laser operating at a wavelength of 1.06 micron.
Laser welding is popular due to its robustness and manufacturability.
Unfortunately, laser welding also suffers from a number of disadvantages.
One major disadvantage of laser welding involves so-called "post weld shift". In such post weld shift, the laser spot weld contracts during the cooling process, which causes a pulling of an element toward the spot weld.
More particularly, in a typical application, one metal element (i.e., a "part") is being mounted on a second metal element (i.e., a "substrate"). Post weld shift causes the part to move on the substrate. By way of example but not limitation, typical post weld shift with a butt weld is shown in FIG. 6; typical post weld shift with a lap weld is shown in FIG. 7; typical post weld shift with a fillet weld is shown in FIG. 8; typical post weld shift with a hybrid fillet-lap weld is shown in FIG. 9; and typical post weld shift with a butt-lap weld is shown in FIG. 10.
Post weld shift can present a serious problem in the manufacture of electronic devices where part positioning must be extremely precise.
By way of example but not limitation, many optoelectronic devices require various components to be aligned with substantial precision, and post weld shift can disrupt such precise alignment.
By way of further example but not limitation, FIG. 11 shows a fiberoptic element being mounted on a substrate, so that the fiberoptic element will receive and transmit light generated by a semiconductor laser which is also mounted on the substrate. In such a situation, alignment between the fiberoptic element and the laser can be crucial for proper functioning of the complete assembly. In such a situation, it is also common for the fiberoptic element to be lap welded to the substrate. Unfortunately, however, post weld shift can cause the positioning of the fiberoptic element to shift as the various lap welds cool, thereby causing mis-alignment between the parts.
For many optoelectronic devices, such part mis-alignment can effectively ruin the device, thereby decreasing manufacturing yield and increasing production costs.
As a result, a number of different techniques have been developed to counteract the effect of post weld shift.
One such technique involves the use of so-called "symmetrical welding". Here, two or more lasers, of equal power, simultaneously effect the laser welding from different directions, so as to reduce the magnitude of any part movement due to post weld shift.
Unfortunately, however, not all types of laser welds lend themselves to such symmetrical welding. See, for example, FIG. 11, where lap welds are used to fasten the fiberoptic element to the substrate. In this situation, all of the lap welds are formed using laser beams which emanate from substantially the same direction, i.e., substantially perpendicular to the plane of the substrate. Thus, in this situation, there is no opportunity to use opposed laser beams to counteract the effect of post weld shift.
Another technique for counteracting post weld shift involves the use of so-called "laser hammering". With laser hammering, after the initial welding process has been completed, and the effect of any initial post weld shift has been determined, a selected one or more of the laser beams is then re-activated, at the same weld spots, so as to re-heat the spot weld and thereby "pull" the part toward the spot weld. Through selective use of such laser hammering, mis-aligned parts can sometimes be brought back into alignment.
Unfortunately, however, there are limitations on the extent to which laser hammering can be used to correct post weld shift. More particularly, it has been found that as laser hammering is repeatedly used on a particular spot weld, it becomes progressively more and more difficult to move the part, until eventually the effect of laser hammering on a particular spot weld becomes negligible.